Rename Devise::Mailer.translate() to something more inline with what it does

I found the translate() method in devise mailer by accident, trying to make a generic translate helper in my own mailer. Turns out the one in devise only tries to figure out a subject and doesn't handle anything more than that, so I believe it's best if it were named appropriately.

There is also a few helper methods in there that take devise_mapping as an argument when they don't really need to since there is an instance variable and helper containing that info already.